This invention relates to a digital intervalometer and more particularly to apparatus operative to generate a digital vernier measurement providing a resolution substantially finer than one clock period.
Digital time interval measuring systems of various types are known in the art. Such systems typically count the number of cycles generated by a continuously operating oscillator during the interval between "start" and "stop" signals. The frequency of the oscillator is usually relatively high with respect to the expected interval duration, but there still exist certain errors and uncertainties correspondng to the timing of the start and stop with respect to the counting interval.
In applications where good time resolution is required, some method must be provided to account for the possible errors at the beginning and end of the counting interval. For illustration, it may be noted that, if the interval of time to be measured is not an exact multiple of the period of the oscillator, an error in exact measurement necessarily exists. This error can be reduced by increasing the oscillator frequency; but the extent to which this expedient may be used is, in turn, dependent upon the maximum speed at which available and economical counters and gating circuits can operate, and also upon the accuracy of the gating circuits used in the system. It will be apparent that the error may easily approach one complete cycle of the oscillator frequency. Therefore, when measuring time intervals in the range of microseconnds or smaller, there is a need for a device which will provide an accurate vernier measurement in digital form.
One system utilized in the prior art to measure the error-producing time periods, which may be referred to as vernier times, is to use separte time-to-amplitude converters to convert the start and stop vernier times to amplitudes and separate analog-to-digital converters to convert the amplitudes to digital values for combination with the clock count accumulated during the measured interval. Each time-to-amplitude converter is on from receipt of its corresponding start or stop pulse until a subsequent clock pulse. After conversion to digital values, the start value is added to the count of pulses during the interval and the stop value is subtracted from the sum. The use of separate time-to-amplitude converters and separate analog-to-digital converters contributes to the complexity of the system.
Among the several objects of the present invention may be noted the provision of a digital intervalometer of high accuracy; the provision of such a system which provides a vernier measurement giving a resolution substantially finer than one clock period; the provision of such a system which is highly reliable and is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.